1. Field of the Invention
The present invention relates to an eccentricity measuring method and is suitable to measure eccentricity of a lens system such as a zoom lens, which is composed of a plurality of lens elements and has a movable portion.
2. Related Background Art
Conventional eccentricity measuring methods are described in detail in “Eccentricity Measurement of Optical System”, Yoshiya Matsui and Masayuki Usui, Optical Technology Contact, Vol. 13, No. 11, pp. 11-20. The following three methods,    (1) a test lens rotation method,    (2) a test lens still East Germany Zeiss method, and    (3) a test lens still image rotator method are introduced.
According to any of those eccentricity measuring methods, an index is projected to the apparent center of curvature of a surface to be tested by an auto-collimation method. An eccentric quantity is calculated from states (deflection and displacement) of a reflection image on the surface to be tested relative to a measurement reference axis.
The method (1) has an advantage that high precision measurement can be performed with a simple structure. However, when a test lens includes a movable portion as in a zoom lens, accurate eccentric measurement is impossible. This reason is as follows. In a lens system including a movable portion, it is necessary to provide a minute gap at a sliding portion which is moved. When the eccentricity of the test lens is measured during the rotation thereof as in this method, an eccentric state is changed by a change in posture due to the weight of the lens itself.
In the measurement using the method (2), the test lens is not rotated. Therefore, it is possible to measure the eccentricity of the lens system which includes the movable portion. However, measurement light transmits through a half mirror six times, so that a light quantity loss becomes a large problem. An application of this method is proposed in JP 03-054287 B.
According to the method (3), the use of an image rotator makes it possible to measure the eccentricity of the test lens without the rotation thereof. In addition to this, since the number of times in use of a half mirror is small, a light quantity loss is low. Applications of this method are proposed in JP 51-009620 B, JP 07-081931 B, JP 07-039982 B, and JP 2621119 B.
A method of performing eccentric measurement by relatively moving an auto-collimation optical system and the test lens is also proposed in JP 04-190130 A.
When the eccentricity of the lens system composed of the plurality of lens elements is measured by the conventional methods, it is particularly important to accurately measure the state of the reflection images on all surfaces composing the lens system. If a reflection image on any of the surfaces cannot be measured, a result of eccentric quantities measured on surfaces located in the back of such a surface becomes inaccurate.
However, when it is intended to actually perform the eccentric measurement by the above-mentioned conventional methods, the state of the reflection image cannot be measured in many cases. More specifically, the following problems occur.    An unmeasurable surface on which the reflection image cannot be measured is caused in some cases.    An eccentric measurement value on a test surface having a large eccentric error includes a large measurement error.
As is apparent from experiments, the test surface having the large eccentric error is likely to become an unmeasurable surface. In order to prevent a measurement error on the test surface having the large eccentric error from increasing, the following method is proposed in JP 07-081931 B. According to the method, the eccentricity of the lens system is temporarily measured. An optimal optical axis is calculated so as to reduce the root-mean-square value of the measured eccentricity. A mounting position of the test lens is adjusted based on a result obtained by the calculation. The eccentricity of the lens system is measured again. However, up to now, the analysis of fundamental causes of the above-mentioned problems and specific measures against the causes are not taken.